The invention relates to a synchronous machine comprising a rotor, which is mounted in two magnetic bearings against a stator having stator windings and has a rotor shaft and, fastened thereto, a pole core in which magnetic field-generating excitation coils are arranged, which either interact with the stator windings to generate rotation of the rotor or serve to generate current in the stator by rotation of the rotor, each bearing comprising a first bearing part which is arranged on or in the stator and is cooled by means of a cryostat and has a high-temperature superconductor structure, and a second bearing part on the rotor side, the first bearing part interacting during operation of the bearing with the magnetized second bearing part which is separated from the first bearing part by a gap, in order to act as a bearing.
The rotor shaft of such synchronous machines, for example motors or generators, are either set in rapid motion by the interaction between the stator windings and the excitation coils, or otherwise rotated so that a current is induced in the stator windings. In order to allow. maximally wear-free, passive bearing, the rotor shaft is mounted in two high-temperature superconductor magnetic bearings, which lie for example next to the pole core in the radial direction.
Such high-temperature superconductor bearings comprise a first and a second bearing part, the first bearing part comprising a high-temperature superconductor structure i.e. a bulk high-temperature superconductor material, which is cooled to below the critical temperature (Tc) by means of a cryostat. In the synchronous machine of the type mentioned in the introduction, this first bearing part is arranged on the stator and interacts with a second bearing part, which is separated from the first bearing part by a gap. The second bearing part is magnetized at least during operation of the bearing, the magnetic field being introduced into the gap. Radial movement of the second bearing part therefore leads to a variation of the magnetic field inside the high-temperature superconductor structure. Since superconductors are however (approximately) ideal conductors, a reaction force is created which attempts to prevent or reverse this magnetic field variation. The second bearing part, and therefore the rotor shaft, are thereby held in the position originally set.
For example permanent magnets, in particular made of NdFeB, may be used for the second bearing part. The magnet quality, in particular the maximum magnetic field achievable by such permanent magnets, is however restricted, so that the supporting force and performance capacity of the high-temperature superconducting bearing is limited. Furthermore, the materials of the permanent magnets, in particular NdFeB, are often found to be brittle so that the maximum achievable rotation speed is also restricted. To this end it has been proposed to provide additional bandages on the second bearing part, although these disadvantageously constitute an additional component which also increases the distance between the first bearing part and the second bearing part.
As an alternative, it has been proposed to use superconducting coils instead of the permanent magnets in the second bearing part. These superconducting coils must however be additionally cooled, so that additional cost is entailed and additional friction losses occur since the refrigerant and a current connection must be fed to these coils.